It is already known practice to create security zones on security paper, and in particular on banknotes, by applying images in the form of a film, label or ribbon, so as to make these papers difficult to falsify, particularly to reproduce by the use of photocopiers, the quality of reproduction of which is ever increasing. These images are often optically variable images comprising either a kinegram or a hologram, which has the property of changing appearance, depending on the angle from which they are viewed. These images may be applied either by hot or cold sealing. Machines for applying such images onto otherwise printed banknote sheets are for example described by EP 0625466 or U.S. Pat. No. 6,263,790 or U.S. Pat. No. 6,302,016. Whereas falsifications by means of simple color photocopiers are thereby no longer possible, the affixing of such images does not eliminate falsifications by forgers able to get hold of holograms and the like.
Usually, the identity marking of a security paper comprises a serial number printed on the document. In order to improve the security effect of the usual serial numbers, EP 0768189 teaches to associate an additional alphanumerical security feature to the serial number, borne by a foil or label, which is attached to the security paper by means of a process as mentioned above. According to the process taught by EP 0768189, the information of the foil or label is read, after the fixing step of the foil onto the security paper, by a reading device. The reading device commands a printer, which prints the same information at another place of the security paper, for example in association with the serial number. The identity marking become thus a composite marking, the reproduction or copy of which is more difficult than with the usual serial number alone or with the serial number associated to a hologram image which does not change from paper to paper. This known proposal, however is not quite satisfactory, since each printing process, like the second printing taught by EP 0768189, leads to some misprints, the number of which is low in reliable equipment but never absolutely nil. But users of security documents wish continuously numbered series, and this can not be guaranteed by a marking process comprising the passage through two successive machines. For the same reason, serial numbers are not printed double-sided on banknotes.
Document WO 98/36913 discloses a method of marking a transparent window in a security document, made of a polymeric substrate, substantially transparent to a selected laser radiation. The substrate is covered double-sided with an ink absorbing the same radiation, and submitted to said radiation along a marking path, whereby ink on both sides is ablated along said path. The mark appears as a transparent window. A drawback of this technique is that both markings are necessarily in register and that whereas from one side, an alphanumerical sequence can be read, from the other it is not readily readable, since it is the mirror image of an alphanumerical sequence. A similar solution is disclosed in U.S. Pat. No. 6,505,779.
EP 0737572 and WO 03/099579 disclose marking systems comprising typographic, inkjet or laser printers positioned radially relative to a sheet fed drum for printing serial numbers, bar codes and additional security features. All said features are printed on the same side of the security document facing the printers.